The present invention relates to a ferromagnetic resonator utilizing ferromagnetic resonance and suitably applicable to microwave equipments such as, for example, microwave filters and microwave oscillators.
There has been proposed a ferromagnetic resonator such as, for example, a filter, utilizing the ferromagnetic resonance of a ferrimagnetic yttrium-iron-garnet (hereinafter abbreviated to "YIG") thin film device formed by growing an YIG thin film through a liquid phase epitaxial growth process (hereinafter referred to as "LPE process") on a gadolinium-gallium-garnet (hereinafter abbreviated to "GGG") substrate, and by selectively etching the YIG thin film in a predetermined pattern. Filters of such a kind are disclosed, for example, in U.S. Pat. No. 4,547,754.
The microwave equipment such as a filter employing such an YIG thin film device has advantages in that the Q of resonance in the microwave band is high, the construction is compact, the LPE process and the lithographic selective etching process is suitable for mass-production, and the use of a thin film facilitates forming microwave integrated circuits employing microstrip lines as transmission lines.
As is well known, it has been usual to use YIG single crystal spheres for the ferromagnetic resonator of a microwave equipment utilizing ferromagnetic resonance. The YIG single crystal ball has advantages in that a magnetostatic mode is difficult to establish and the single resonance mode is established in a uniform precession mode. However, the YIG single crystal sphere has problems in processing and mass production. Accordingly, the development and practical application of the ferromagnetic resonator employing a YIG thin film, namely, a ferrimagnetic thin film, has been desired.
Incidentally, the magnetostatic mode established when a DC magnetic field is applied perpendicular to the surface of a ferrimagnetic disk is analyzed in Journal of Applied Physics, Vol. 48, pp. 3001-3007, July, 1977, in which modes are represented by (n, N)m, where n is the number of nodes along the circumferential direction, N is the number of nodes along the diameter, and m-1 is the number of nodes in the direction of thickness. When the high-frequency magnetic field is satisfactorily uniform over the entire range of the ferromagnetic disk, modes of (1, N).sub.1 are principal magnetostatic modes. In constructing a microwave filter or a microwave oscillator, the main mode (1, 1).sub.1 of the (1, N).sub.1 system is employed and the rest of the magnetostatic modes are regarded as spurious modes, namely, spurious response or spurious oscillation. For example, the aforementioned U.S. Pat. No. 4,547,754 proposes a resonator employing a ferrimagnetic YIG thin film provided with an annular groove, and a resonator employing a ferrimagnetic YIG thin film having a central portion of a thickness smaller than that of the peripheral portion thereof, both designed to avoid the spurious response mode.
On the other hand, since the operating frequency of the ferrimagnetic thin film resonator can be varied over a wide range by varying the magnetic field to be applied thereto, the ferrimagnetic thin film resonator is applied, for example, to variable-frequency microwave oscillators and variable-frequency microwave filters. In such application, however, the unloaded Q of the spurious mode increases together with the unloaded Q of the main mode with frequency, and hence the spurious mode cannot be ignored. Such a behavior of the ferrimagnetic thin film resonator is due mainly to the distribution of the exciting magnetization.
As shown in FIG. 23 by way of example, in the exciting method shown in U.S. Pat. No. 4,547,754, a strip line, namely, a transmission line 3, having one end connected to a grounding conductor 2, and having a uniform thickness, a uniform width and a uniform impedance is disposed across a disk-shaped ferrimagnetic thin film 1 so as to be coupled magnetically with the ferrimagnetic thin film 1. Supposing that the direction along the transmission line 3 is an x-direction, the direction along the surface of the ferromagnetic thin film 1 and perpendicular to the x-direction is the y-direction, the distance between the grounded end of the transmission line 3 and the ferrimagnetic thin film 1 is l.sub.1, and the length of a portion where the ferromagnetic thin film 1 and the transmission line 3 overlap with each other is l.sub.2, a magnetic field Hy generated by a current i.sub.rf along the y-direction is substantially uniform when l.sub.1 .ltoreq.x.ltoreq.l.sub.1 +l.sub.2.
Calculated distributions of magnetization for modes (1, N).sub.1 (N=1, 2 and 3) over the ferrimagnetic thin film 1 in the state of magnetic resonance are shown in FIG. 24. These distributions of the magnetization are the same with respect to any diametrical direction.
In the consideration of the magnetization distribution of the magnetic field applied to the ferrimagnetic thin film 1 in this construction, when a high-frequency current i.sub.rf is supplied, a standing wave Ix is expressed by EQU Ix=i.sub.rf cos (2.pi.x/.lambda.g) (1)
where .lambda..sub.g is the wavelength on the transmission line 3. When the y-component of the magnetic field generated by the current i.sub.rf is expressed by Hy(x), Hy(x) .varies. Ix. That is, EQU Hy(x) .varies. i.sub.rf cos (2.pi.x/.lambda.g) (2)
Therefore, at a position x&lt;&lt;.lambda..sub.g /4, namely, a position near the grounded end of the transmission line 3 where x is nearly zero, Hy(x) is practically constant. In a range where x.ltoreq..lambda..sub.g /4, Hy diminishes along a cosine curve to zero at x=.lambda..sub.g /4.
Thus, when the frequency of i.sub.rf is low, namely, when .lambda..sub.g is comparatively large, Hy is substantially constant along the transmission line 3, and, when the frequency of i.sub.rf is comparatively high, namely, when .lambda..sub.g is comparatively small, the grounded end and opposite end of the ferromagnetic thin film 1 are different in the intensity of magnetic field from each other.